The present invention relates to the removal of oil, solids and other waste from a liquid and, in particular, to the removal of scum from a liquid surface in a gas flotation process.
In certain instances it is necessary to separate waste matter from a waste liquid in which the waste matter is emersed, e.g., the removal of solids or oil from water. This can be achieved by means of a so-called gas flotation process in which a gas, such as natural gas for example, is introduced into the liquid. The gas is atomized and the bubbles adhere to solids and float them to the liquid surface where they collect as a froth or scum. Skimmer blades disposed at the surface level are rotated to push the scum into an external conduit for disposal. Such a system is disclosed, for example in O'Cheskey et al U.S. Pat. No. 3,972,815 issued Aug. 3, 1976.
In particular, it has been common to deliver the waste liquid to a cell and force the gas through the bottom of a conduit which projects downwardly into the liquid. The gas can be drawn into the conduit by means of a Venturi-type nozzle as treated waste water is circulated through the conduit. The gas is drawn from a gas blanket maintained above the liquid surface in the cell. The gas blanket, which prevents oxidation within the tank, is circulated at low pressure by means of a gas blower.
The skimmer blades can be in the form of rotary paddles which are rotatably mounted on motor-driven horizontal axles located at opposite sides of the cell. The paddles can be mounted for vertical adjustment so as to be adaptable to different liquid surface levels.
In certain cases the scum which is expelled can constitute a valuable substance, such as oil, which can be subsequently sold as a petroleum product. However, since the skimming paddles expell a considerable quantity of liquid along with the oil scum, e.g., the expelled substance may comprise 90% liquid, the substance must be treated in order to separate the oil from the liquid. In addition, the paddles themselves are relatively expensive to purchase, operate and maintain, and are difficult to adjust vertically.
It is, therefore, an object of the present invention to minimize or obviate shortcomings of the types discussed above.
Another object is to provide a gas flotation apparatus in which the expelled scum contains little, if any, liquid.
A further object is to provide a scum expelling mechanism which is relatively inexpensive to install, operate and maintain.
Another object is to provide a scum expelling mechanism which is easily adjustable to accommodate different liquid levels.